Active Devices Flashcards
Silicon Dioxide
Good insulator
High permittivity
Large bandgap between the valence and conduction band
Stable molecule and can tolerate high temperatures
Resistant to many chemicals make it ideal for use in fabrication of semiconductor devices
Transistors
Three terminal semiconductor devices that can regulate current and voltage and can also act as a switch
What are the 3 terminals in a Field Effect Transistor
Drain, Source and a Gate
Gate is the control input
Basic operation of FETs
A voltage on a control input produces an electric field that affects the current between two other terminals
MOSFET
A metal oxide semiconductor field effect transistor - a FET with an insulated gate
Constructed with a body made of a P-type substrate with holes as its majority carriers
Two n type channels are etched into the top surface with electrons as the majority carriers - these become the source and the drain when metal electrodes are attached
A gate electrode is insulated from the region between the source and the drain by the gate oxide - Silicone dioxide insulating layer
Source electrode is connected internally to the body electrode
MOSFET Operating Principles
A depletion layer is formed at the P-N junction surrounding source and the drain
Depletion layer acts as an insulator between the source and the drain
When a potential is applied between the source and the drain electrode - the depletion layer restricts the flow of charge carriers and no current passes through the transistor
MOFSET is a voltage controlled current source
The silicon dioxide insulating layer metal oxide layer gives the MOSFET an extremely high input resistance - so high that the MOSFET draws neglible current from the input gate signal
MOFSET - Positive field at gate eletrode
V gs > V th - A positive voltage bias at the gate electrode sets up a positive electric field
I ds > 0 A - Current flows between the source and the drain via the inversion layer
I gs ~ 0 A
N Type minority carriers (electrons) attracted by the positive field migrate to the gate electrode and form a conductive N channel inversion layer between the source and the drain
MOFSET - No Field at Gate
When V gs = 0V - the field surrounding the gate electrode collapses and no current (I ds) flows between the source and the drain
I gs ~ 0 amp
I ds = 0 amp
V gs < V th - no field at gate electrode
When the gate field is turned off - the N type minority scatter throughout the p type substrate and collapse the inversion layer
MOFSET - Gate voltage is much greater than the threshold voltage
I ds»_space; 0 Amp - Larger current flows between the source and the drain
V gs > V th
A larger positive voltage bias at the gate electrode attracts more N-type minority carriers to the inversion layer
A larger inversion layer allows for a larger current to pass between the source and the gate
MOSFETs Operation Overview
MOSFET is a type of transistor that works by varying the width of a conducting N channel (inversion layer) along which charge carriers flow
3 Terminals - Gate, Source and Drain
In an n channel (NPN) Mosfet - charge carriers are electrons - enter at the source and exit via the drain
Conventional current is from drain to source (I ds)
The gate voltage (V gs) controls the thickness of the channel
A positive gate voltage larger than the threshold voltage (V th) attracts electrons towards the gate via the electric field generated
The greater the electric field - the thicker the inversion layer becomes - thus reducing the resistance between the drain and the source - channel is said to be enhanced and will allow a greater current (I ds) to flow
Creating a negative field at the gate electrode repels electrons increasing the resistance of the channel and reducing the current - channel is said to be depleted
Applying an input signal voltage to the gate (V gs) controls the drain source current (I ds) and hence the output in the external circuit
The insulating metal oxide layer gives the MOSFET an extremely high input resistance - so high that the MOSFET draws neglible current from the input signal
It draws very little power from the input signal when operating as an amplifier
Metal oxide layer is extremely thin - MOSFET is susceptible to destruction by electrostatic charges building up on the oxide layer between the gate and the source (behaves like a capacitor) - gate should never be left unconnected - a path to ground is needed to allow the charge to flow off
P Channel MOSFET
Type of MOSFET in which the channel of the MOSFET is composed of a majority of holes as charge carriers
When the MOSFET is activated and is on, the majority of the current flowing are holes moving through the channels
MOSFET Output Characteristics when V gs < V th
When V gs is less than V th - MOSFET is off because there is conducting channel
Small leakage current flows, of the order of a few nanoamps
Above V th a channel starts to form and the MOSFET turns on
I ds versus V ds characteristic curves have almost vertical and almost horizontal parts - linear almost vertical part of the curves correspond to the ohmic region - were the MOSFET channel acts like a resistor
Linear region - above V th the drain current I ds increases slowly at first with an increase in V gs and then much more rapidly
The horizontal part corresponds to the constant current region - where there is almost no increase in drain current for increasing V ds - this is the saturation region - drain-source current is then controlled by the value of V gs
Summary of operating regions
Cut off region - with V gs < V th - gate-source voltage is lower than threshold voltage - so the MOSFET is switched off and I ds = 0 - MOSFET acts as if it was an open circuit
Ohm’s Law Region - with V gs > V th - the MOSFET acts like a variable resistor whose value is determined by the gate voltage V gs - up to the point where it becomes saturated
Saturation Region - with V gs > V th and a high enough value of V ds - the MOSFET is in its constant current region and is switched fully on or saturated - the current I ds is at its maximum and depends on the value of V gs
Transconductance (g m)
measured in units of mAV^-1
Change in drain current caused by the change in the voltage between the gate and the source
Calculating the gradient of the ohmic part of the graph
g m = Change in I d / change in V gs
Breakdown Region
Above saturation region is the breakdown region
At a certain value of V ds called the breakdown voltage - drain-source path of the MOSFET breaks down internally and a large current will flow - destroying the transistor
Application of MOSFETS
Switches
